The present invention relates to a production method of a ultra fine grain steel. More specifically, the invention relates to a method of producing a ultra fine grain steel useful as a welding steel having a high strength.
Hitherto, a controlled rolling-accelerated cooling technique is an effective method for obtaining fine ferrite in a low-alloy steel. That is, by controlling a cumulative percentage of reduction in an austenite non-recrystallization region and the cooling rate thereafter, a fine grain has been obtained. However, the ferrite grain size obtained is at most 10 xcexcm in an Si-Mn steel and at most 5 xcexcm in an Nb steel as the limits. Furthermore, as described in Japanese Patent Publication Nos. 39228/1987 and 7247/1987, it is reported that by adding a reduction that the sum total area reduction ratio of 75% or higher in the temperature range of Ar1 to Ar3+100xc2x0 C. including a 2-phase region and thereafter cooling at a cooling rate of at least 20 K/second, ferrite grains having grain sizes of from about 3 to 4 xcexcm are obtained. However, as is mentioned in, for example, Japanese Patent Publication No. 65564/1993, for obtaining ferrite grains having grain sizes of smaller than 3 xcexcm, very large reduction amount and cooling rate (at least 40 K/second) are required. Quenching of the cooling rate of at least 20 K/second is a means capable of being realized only in the case of a thin sheet thickness and cannot be realized for the production method of steels for general welding structure, which is widely and practically used. Also, with regard to strong working itself, in roll rolling, it is generally difficult to carry out a large reduction exceeding 50% at an austenite low-temperature range because the extent of the deformation resistance and the restriction on one pen rolling. Also, for the cumulative reduction in a non-recrystallization region, at least 70% of reduction is necessary in general, which is also difficult because of the temperature lowering of steel sheet.
It is known that a ferrite grain structure of a control-rolled steel generally has an strong texture, and the ferrite grains obtained as the result of a strong reduction becomes to have a small angle grain boundary. That is, by simple strong working, an strong texture is formed and ferrite grains made of a large angle grain boundary cannot be obtained. Accordingly, even when strong working higher than those shown in Japanese Patent Publication Nos. 39228/1987 and 7247/1987 is carried out, it is difficult to obtain a fine ferrite grain structure made of a large angle grain boundary.
Under such circumstances, the present inventors previously developed methods of obtaining a ultra fine grain steel made of ferrite having a mean grain size of not larger than 3 xcexcm as the base phase, after austenitizing raw materials by heating to a temperature of at least an Ac 3 point, applying compression working of the reduction ratio of at least 50% at a temperature of at least the Ar 3 point and cooling (Japanese Patent Application Nos. 256682/1997, 256802/1997, and 52545/1998). By this new production methods, it becomes possible to provide a ultra fine grain steel made of, as the base phase, ferrite having a mean grain size of not larger than 3 xcexcm and surrounded by a large angle grain boundary of an Disorientation of at least 15xc2x0.
However, a further improvement has been practically desired on these new methods. This is because it is desirable to obtain a finer grain and also, from the industrial viewpoint, the deformation resistance at hot working is desirably as low as possible. In particular, when working of at least 50% is carried out in an austenite low-temperature range by one pass, the deformation resistance is large and it is desirable to lower the resistance as low as possible. That is, in regard to obtain ferrite having a mean grain size of not larger than 3 xcexcm, and preferably not larger than 2 xcexcm as the main phase by working at an austenite low-temperature range and control cooling, it can be said that a new method capable of producing a ultra fine grain steel made of ferrite having a mean grain size of not larger than 3 xcexcm, preferably not larger than 2 xcexcm as the main phase under a lower deformation resistance, by a less reduction amount, and by a particularly slow cooling rate has been required.
The present invention has been made under the circumstances as described above and to provide a new method of producing a ultra fine grain steel made of ferrite having a mean grain size of not larger than 3 xcexcm, preferably not larger than 2 xcexcm as the base phase under a lower deformation resistance, by a less reduction amount, and by a particularly slow cooling rate.
That is, a 1st aspect of the invention provides a method of producing a ultra fine grain steel made of ferrite having a mean grain size of not larger than 3 xcexcm, after ingoting raw materials, by austenitizing the ingot by heating it to a temperature of at least an Ac 3 point, then, applying compression working of at least a reduction ratio of at least 50% at a temperature of from an Ae 3 point or lower to an Ar 3 point xe2x88x92150xc2x0 C., or to a temperature of at least 550xc2x0 C., and thereafter, cooling, wherein the strain rate as compression working is in the range of from 0.001 to 10/second.
A 2nd aspect of the invention provides the ultra fine grain steel made of ferrite having a mean grain size of not larger than 2 xcexcm as the base phase produced by the method described above.
A 3rd aspect of the invention provides the production method of the aspect 1 wherein the strain rate is in the range of from 0.01 to 1/second.
Also, a 4th aspect of the invention provides the production method of the aspect 1 wherein the cooling rate after working is not higher than 10 K/second.